The present invention relates generally to motor control and, more particularly, to a motor drive with synchronized timing.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Rotating motors are typically controlled by a motor drive that receives reference motor velocity and position signals and produces a torque signal that is applied to the motor. The torque signal is generally controlled using a pulse width modulated (PWM) technique. Adjustment of the torque signal based on changes to the reference velocity or position signals relative to measured feedback velocity and position signals ensures that the motor rotates at the reference velocity.
Some applications require precise motor control across multiple, synchronized motors. For example, an electronic line shaft may be employed in a printing application to move the paper or other material over rollers and through various stages of the printing process. Typical printing processes employ multiple colors, each applied at different locations along the line. Hence, to ensure print quality, the various stages are synchronized. A lack of synchronicity between the stations results in misregistration between the colors, leading to unacceptable product that may need to be scrapped.
Previous generations of printing technology employed a mechanical line shaft mechanically linked to the various printing stations. Rotation of the line shaft by an electric motor activated rollers and other printing station tools along the line to conduct the printing process. In a mechanical line shaft system, factors such as play in the mechanical linkages, stretching of the paper web, and torsional flexing of the line shaft itself make it difficult to achieve and maintain synchronicity between the printing stations, especially during periods of acceleration and deceleration of the printing system. It has been observed that when synchronicity is not maintained, product generated includes excessive flaws and is often unacceptable for intended use. Mechanical line shafts also have reduced flexibility in addressing print changes. Hence, where changes are required, down time may be excessive.
More modern printing systems, commonly referred to shaftless printing systems or electronic line shaft systems, employ a plurality of motors and associated rollers that are electrically synchronized, as opposed to mechanically synchronized. Lack of synchronicity in an electronic line shaft results in similar problems, such as color misregistration, evident in a mechanical line shaft system.
When operating a plurality motors synchronously in an automated system, several factors exist that may cause the position of the motors to deviate from each other even though they are all operating pursuant to a single reference velocity signal. For instance, motor inertia between motors at different stations is often non-uniform and can cause one motor to drift from the other motors.
Typical motor drives for controlling motors are implemented using software executed by one or more central processing units (CPUs). As CPU clock rates have risen, so too has the control bandwidth available to a motor drive. However, higher control bandwidth does not necessarily equate to higher performance. To this end, as bandwidth increases, so does the susceptibility of a motor drive to noise which can lead to operation, rattles, clunks, tendency to resonate, lack of robust performance, etc. In fact, in many cases, the noise level that results from operating a drive at a maximum bandwidth associated with high CPU clock cycles, instead of increasing control performance has been known to degrade performance appreciably. In this regard, most processes have an ideal operational bandwidth that is much lower than the high bandwidth associated with high speed CPU clock cycles. For example, an ideal operational bandwidth may be one or two orders of magnitude less than the bandwidth associated with high CPU clock cycles.
Position errors in a drive system are controlled by a position regulator that acts on the difference between a reference position and a feedback position determined using a position feedback device such as, for instance, an optical encoder. That difference is commonly referred to in the motor control industry using terms such as “following error”, “tracking error”, and “position error”. A key performance measure of a position regulator is to quantify regulator tracking (i.e., how close to zero can the error be maintained under specific conditions). Typically, tracking is evaluated under two such conditions, steady state velocity, and acceleration/deceleration.
Position error in a real system contains a noise component with zero average value and a “DC component” that may or may not be zero. The DC component may be referred to as simply “position error”. Under steady state velocity conditions position error can readily be held to zero using techniques that are well understood in the industry. However, in applications where a high degree of precision is required and periods of acceleration and deceleration occur, known techniques of minimizing the position error have been less successful.
In typical motor drives the tasks for generating reference signal updates, position updates, velocity updates, and PWM updates to the torque signal are processed discretely, and at different update intervals. For example, the PWM task typically has the highest update frequency, followed by the velocity task, and finally by the reference and position tasks. Besides having different update frequencies, the tasks may be performed by different processing units (e.g., microprocessors) within the motor drive that operate asynchronously and at different clock speeds.
Because the various tasks are not synchronized, a more frequently occurring task, such as the PWM update, may operate on data that is relatively stale during certain cycles. For example, where the PWM update happens to occur immediately after a velocity update, the data is most current. Because the PWM and velocity tasks execute asynchronously, during subsequent updates the time interval between the last velocity update and the current PWM update will vary, a phenomenon commonly referred to as a beat frequency, which occurs as the signals move in and out of phase. This beat frequency can give rise to a noticeable error in a highly sensitive application, such as printing.